Widespread exposure to fluoride has been a major factor in the caries decline among American children, but it has also led to an increase in dental fluorosis. While fluorosis is not considered a public health problem today, its prevalence and severity must be controlled to avoid any future public backlash against fluoride use. To maximize the cariostatic effects of fluoride while minimizing fluorosis in an environment of multiple fluoride exposure, the contribution of various modes of fluoride use to both cariostasis and fluorosis needs to be better understood. The clinical trials to answer these issues cannot be conducted today because of ethical concerns, but "opportunistic" epidemiology provides a quasi-experimental design to study fluoride effects in humans. Such an opportunity arose when the water supply for Durham, North Carolina, normally fluoridated at 1.0 parts per million (ppm), was at zero or sub-optimal fluoride levels between September 6, 1990 and mid-August 1991 because of equipment failure. The aim of this study is to measure the impact of the II-month break in water fluoridation on the development of dental caries and fluorosis. The research design is a follow-up cohort comparison with a nested case- control study, which will analyze clinical and interview data from 6 cohorts of schoolchildren in the elementary grades of Durham Public Schools. The water supply from the City of Durham extends well outside city limits, so eligible children will be determined by checking their addresses against the map of the water supply area. The cohorts (n=238) will be established to relate the interruption in fluoridation to developmental periods (secretion, pre-eruptive maturation, immediate post-eruption) for specific teeth. In order to examine children in each cohort when they are at the same age, 3 annual examination cycles are planned in the autumn of each year during a 3-year study. The two examiners will record caries and fluorosis. Interviews for demographic information and fluoride histories will be carried out by a trained interviewer during the first two years. Comparison of results between different cohorts will pen-it conclusions on the effects of water fluoridation at different periods of tooth development, and the case- control analysis will provide quantification of the risk. Results from studying this "window" in water fluoridation will permit a clearer view of the impact of water fluoridation in modern conditions of multiple fluoride exposure, and will be of value in the further refinement of protocols to maximize fluoride benefits while minimizing fluorosis